A prior art electronic tuning type FM radio receiver will be described with reference to FIG. 7. A signal received by an antenna 1 is supplied to a high frequency amplifying circuit RA. This high frequency amplifying circuit RA is formed of high frequency tuning circuits 2 and 4 that are provided before and after a high frequency amplifier 3. Each of the high frequency tuning circuits 2 and 4 comprises, for example, a multiple tuning circuit and is formed of a pair of electromagnetically-coupled windings L1 and L2, a capacitor C connected in parallel thereto and a series circuit formed of a pair of voltage-controlled type variable capacity elements (varactor diodes) VC and connected in parallel to the respective winding. The high frequency signal from the high frequency amplifying circuit RA is supplied to a mixing circuit 6 which forms a frequency converting circuit 5. The intermediate frequency signal obtained from the mixing circuit 6 is supplied through an intermediate frequency signal amplifying circuit, that is, an intermediate frequency amplifying circuit 8 to a demodulating circuit, that is, a frequency detecting circuit 9. Reference numeral 10 designates an output terminal at which a frequency-detected output is developed. Reference numeral 7 designates a local oscillator which forms a part of the frequency converting circuit 5 and which includes a resonance circuit 7A which is formed of a winding L, a capacitor C connected in parallel thereto and a series circuit formed of a pair of voltage-controlled type variable capacitor elements (varactor diodes) VC and connected in parallel to the winding.
Reference numeral 11 designates a PLL (phase locked loop) employed as the tuning control voltage generating circuit and which comprises a reference oscillator 12, a phase comparator 13, a programmable frequency divider 14 and a low pass filter 15. The oscillation signal from the local oscillator 7 is supplied to the programmable frequency divider 14 and thereby divided by N. The signal thus frequency-divided and the reference signal from the reference oscillator 12 are supplied to the phase comparator 13 and thereby phase-compared. Then, the compared output is supplied to the low pass filter 15.
The tuning control voltage obtained from the low pass filter 15 is supplied to the cathodes of the respective voltage-controlled type variable capacity elements VC in the resonance circuit 7A of the local oscillator 7. Further, this tuning control voltage is supplied to the respective cathodes of the respective voltage-controlled type variable capacity elements VC of the respective high frequency tuning circuits 2 and 4 in the high frequency amplifying circuit RA.
In such electronic tuning type FM radio receiver, a channel selection frequency is determined by selecting the frequency-dividing ratio of the programmable frequency divider 14 in the PLL 11. In other words, when the frequency-dividing ratio of the programmable frequency divider 14 is altered, the oscillation frequency of the local oscillator 7 is altered and the center band pass frequencies of the high frequency tuning circuits 2 and 4 in the high frequency amplifying circuit RA are altered to become substantially coincident with the channel selection frequency.
In the above mentioned prior art electronic tuning type FM radio receiver, the center pass band frequencies of the high frequency tuning circuits 2 and 4 in the high frequency amplifying circuit RA are altered so as to become substantially coincident with the channel selection frequency, that is, the carrier frequency of the high frequency receiving signal. That is, with respect to the frequency characteristics of the high frequency amplifying circuits 2 and 4 as shown by a curve in FIG. 8, a carrier frequency f0 is coincident with the center band pass frequency thereof. Since this carrier signal is frequency-modulated by a modulation signal (audio signal), the high frequency receiving signal supplied to the high frequency amplifying circuit RA is changed around the frequency f0 from time to time. For example, when a momentary frequency thereof is f0-.DELTA.f, as illustrated in the figure, the frequency response in the high frequency amplifying circuit RA is lowered as compared with a case where the frequency is f0. Also, on the basis of the value .DELTA.f, the phase of the high frequency receiving signal is changed so that the high frequency receiving signal is distorted.
Accordingly, it is an object of this invention to provide an electronic tuning type FM receiver in which a high frequency receiving signal is not easily distorted in the high frequency amplifying circuit.